The present invention relates to a catalyst based on silica and sulphuric acid and its use in the catalytic alkylation of isobutane and/or isopentane by means of at least one olefin, which makes it possible to obtain at least one product e.g. in the group constituted by dimethyl butanes, trimethyl pentanes, trimethyl hexanes and trimethyl heptanes.
It is known that for the supply of internal combustion and controlled ignition engines and in particular those having a high compression ratio, it is particularly interesting to have high-octane fuels, i.e. essentially constituted by highly branched paraffin hydrocarbons. The alkylation of isoparaffins (isobutane and isopentane) b olefins containing 3 to 6 carbon atoms per molecule makes it possible to obtain such products. This reaction requires the use of very acid catalysts, with the aim of reducing unwanted reactions, such as olefin hydride extraction and polymerization reactions, which supply only slightly branched hydrocarbons with a low octane number and unsaturated hydrocarbons, cracking reactions and disproportionation reactions.
Existing processes for the production of hydrocarbons by the alkylation of isobutane by olefins use either sulphuric acid, or hydrofluoric acid as the catalyst. In these processes, the acid catalyst constitutes a liquid phase, which is contacted with the liquid olefin-isobutane mixture to form an emulsion. These processes are expensive and cause serious problems with regards to the safety of personnel and the environment. In order to obviate these problems, different catalytic systems of sulphuric and hydrofluoric acids in the liquid phase have been investigated.
In order to catalyze alkylation reactions of isoparaffins by olefins, a proposal has already been made to develop acid catalysts from numerous acid solids of various types. Among the families of acid catalysts, reference can be made to molecular sieves (cf. e.g. U.S. Pat. Nos. 3,236,762, 3,251,902, 3,644,565, 4,377,721, 4,384,161 and 4,300,015), macromolecular resins, optionally associated with BF.sub.3 (cf. e.g. U.S. Pat. Nos. 3,855,342, 3,855,343, 3,862,258 and 3,879,489), per fluorine resins of the Nafion type (cf. e.g. U.S. Pat. Nos. 4,056,578 and 4,038,213) Lewis and/or Bronsted acids deposited on various inorganic supports (cf. e.g. U.S. Pat. Nos. 3,975,299, 3,852,371 and 3,979,476), chlorinated aluminas (cf. e.g. U.S. Pat. Nos. 3,240,840, 3,523,142, 3,607,859, 3,523,142, 4,066,716, 4,083,800 and 4,066,716) graphites intercalated by Lewis and/or Bronsted acids (cf. e.g. U.S. Pat. Nos. 4,083,885, 4,116,880, 4,128,596 and 3,976,714) and anions deposited on oxide supports such as ZrO.sub.2 /SO.sub.4 (cf. e.g. JP-01288 329, JP-01245953 and JP-61242641). These solids lead to the production of branched isoparaffins, but suffering from several major defects, including the use of often very high isobutane/olefin molar ratios in order to limit the extent of secondary reactions and low stability in time of the catalytic activity (inhibition of the catalyst by the deposition of unsaturated oligomers), so that said catalysts frequently have to be regenerated. Moreover, the limited acidity of certain acid solids, such as e.g. molecular sieves, makes it necessary to use high reaction temperatures, which is prejudicial to the obtaining of high octane hydrocarbons.